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Friday, 25 March 2016

The undignified end to the life of a large (6m wingspan) azhdarchid: being squabbled over by Saurornitholestes and other local ruffians. We know a scene like this must have occurred at least once thanks to an azhdarchid fossil discussed below, and a growing number of specimens are providing similar insights into the place of azhdarchids in Mesozoic food webs.

Discussions of the lifestyles of fossil animals primarily centre around their functional morphology - the use of comparative anatomy and biomechanical calculations to assess their anatomical suitability to certain habits and behaviours. We use these approaches to deduce their likely diets, locomotory strategies, likely interactions with ancient environments and other organisms, and so on. But despite the widespread and proven utility of these techniques, it's widely regarded that these approaches can only provide probabilities and hypotheses about fossil habits. They allow us to predict, but not necessarily know how ancient animals functioned in their respective ecosystems. For the latter, we rely on fossils to provide us with direct records of animal habits - gut content, evidence of animals attacking and ingesting each other, trace evidence of foraging strategies and so on. For vertebrates, these fossils are very rare - hence our general reliance on functional anatomy - and single examples of palaeoecologically-significant fossils can provide crucial insights into ancient lifestyles and ecosystems.

As you might expect, such fossils are particularly rare for pterosaurs. Flying reptiles were not prone to fossilisation at the best of times and this - through simple probability - dramatically lowers their chance of forming fossils with palaeoecological significance. As recently as the 1990s some authors lamented the lack of these fossils and tried to ascribe some significance to their rarity. But renewed interest in pterosaur science, an increased rate of discovery of flying reptile remains and greater alertness for details on fossil specimens has seen our sample size of palaeoecologically-significant pterosaur specimens grow considerably in the last few years. These records have been augmented more in some taxa than others so that our record of pterosaur palaeoecology is looking increasingly patchy: some groups now now have a half-dozen or more of these fossils to their names, others have none at all. There's probably no real significance to this other than the habits and anatomy of some pterosaurs being more suited to recording these fossils than others.

I thought it might be fun to take a look at two pterosaur taxa with increasingly good palaeoecological records. In an upcoming post, we'll look at the really quite excellent palaeoecological data for the Jurassic pterosaur Rhamphorhynchus muensteri, but today I want to discuss the palaeoecology of azhdarchids. That's right: the lifestyles of these sometimes giant, sometimes long necked, always edentulous, and increasingly famous flying reptiles are not only predicted by functional morphology, but can be deduced somewhat from fossils too. Thanks to a number of specimens discovered in recent decades we have an idea what happened to azhdarchids when they died, what sort of animals they must have encountered in day to day life, and maybe even how they foraged.

Meeting the (teeth of the) neighbours

The majority of the azhdarchid palaeoecological record pertains to specimens showing which animals ate them, their teeth and puncture marks being left on azhdarchid bones, or else azhdarchid remnants being found in the torsos of other animals as gut content. One of the most famous of these fossils is an incomplete azhdarchid skeleton from the upper Cretaceous Dinosaur Park Formation of Canada. This fossil is noteworthy for not only being one of the most substantial pterosaur fossils from Canada, but also for the tooth gouges and embedded dinosaur tooth found at the distal end of the tibia (Currie and Jacobsen 1995). The tooth belongs to the 2 m long dromaeosaur Saurornitholestes langstoni, and the size and provenance of the gouges indicate they were probably made by the same species, perhaps the same individual. The size discrepancy between the pterosaur and dinosaur here is pretty dramatic. Check out the size of the tibia of the (c. 6 m wingspan) pterosaur compared to the embedded tooth:

Immature azhdarchid tibia from the Dinosaur Park Formation of Alberta, Canada, with an embedded dromaeosaur tooth (arrowed). Note the size of the pterosaur remains compared to those of the dinosaur. Image by Liz Martin-Silverstone, borrowed from her Twitter account.

This is not the only known occasion of a dromaeosaur interacting with an azhdarchid. Recently Dave Hone and colleagues (2012) described an articulated Velociraptor mongoliensis specimen from Tugrikin Shireh (Gobi Desert, Mongolia) with fragmentary remains of an azhdarchid pterosaur in its gut (below). That this reflects true gut content, and not just chance association of pterosaur bones with those of a dinosaur, is indicated by the articulation of the specimen and location of the pterosaur remains within an envelope of dorsal ribs. It's presumed that the dinosaur was eating bits of pterosaur bone with meat attached, not just swallowing chunks of bone.

Crocodyliforms are also thought to have consumed azhdarchids. Possible puncture wounds from these reptiles are found on the holotype specimen of the Eurazhdarcho langendorfensis, from the upper Cretaceous Sebeş Formation of Transylvania (Vremir et al. 2013). Round bite marks and crushing are found on the cervical vertebrae (below) and distal metacarpal IV of this specimen. In this instance the azhdarchid was unlikely to have been receiving selective treatment by the local reptile fauna, as crocodyliform biting traces are common to many bones from this formation (Vremir et al. 2013).

Azhdarchid bones as an insect hotel?

A further possible record of azhdarchid consumption pertains to strange oval punctures at the back of a Quetzalcoatlus sp. skull (Kellner and Langston 1996). These may record tooth marks, although there is no obvious indication as to what animal might have made them. A rather different take on them was suggested by Kellner et al. (2010), who noted that insect borings had been found the same Quetzalcoatlus material. Kellner et al. suggest that these borings were described by Kellner and Langston (1996) but, to my knowledge, this suggestion is an error as I can find no mention of insect borings in the 1996 paper. I assume this comment provides a different interpretation of the skull punctures, but it might not. I'd really like to know more about this, as there are as yet no confirmed cases of pterosaur bones being utilised by insects. I wonder if this is a simple mistake, or might pertain to details of as-yet undescribed portions of Quetzalcoatlus anatomy?

An azhdarchid foraging trace?

Pterosaur foraging traces are known from a number of Jurassic and Cretaceous tracksites (Lockley and Wright 2003). Most look very similar to those known for birds - paired impressions made in the substrate surfaces by beaks pecking at the ground. Interested parties need only check out the mudflats frequented by wading birds to see analogous traces left by modern fliers. Sometimes we find longer scrapes or gouges made by pterosaur beaks sweeping low across sediments, too. These are sometimes interpreted as tail drags, but it's difficult to envisage sensible scenarios where short-tailed pterodactyloid pterosaurs - those thought to have been creating the pterosaur tracks we know of - randomly sweep their tails across the ground.

Such a beak scrape might be known preserved alongside a possible azhdarchid track from the Late Cretaceous (Campanian) Cerro del Pueblo Formation of North Mexico (Rodriguez-de la Rosa 2003). This track comprises one well-preserved footprint and a series of handprints, alongside several sharp, linear gouges. Other tracks from this locality - known as the El Pelillal tracksite - include turtles, crocodylomorphs, theropods and mammal-like creatures, and the sedimentary setting is considered a shallow, freshwater environment. When described, these pterosaur prints were considered to represent the generic pterodactyloid trace Pteraichnus (Rodriguez-de la Rosa 2003), but the footprint bears little similarity to the broad, triangular-shaped impressions of this ichnotaxon (below). In my 2013 book I argued that the narrow form, pronounced heel impression and short, blunt toes of this print much more reminiscent of Haenamichnus, a trace thought with good reason to represent the footfalls of azhdarchids (below, Hwang et al. 2002; Witton 2013). The age of the specimen is further indication of an azhdarchid identity, the Campanian being a stage of the Mesozoic where azhdarchids seem to largely dominate pterosaur evolution.

The El Pelillal pterosaur trace described by Rodriguez-de la Rosa (2003), argued here and elsewhere to represent an azhdarchid trace (Haenamichnus) rather than a generic pterosaur track (Pteraichnus) - see for yourself in the inset. Illustrations after Rodriguez-de la Rosa 2003 and Hwang et al. 2002.

If this is an azhdarchid track, could the sediment gouges represent marks made by the same animal? The original 2003 description provides little commentary little on these marks, but, to be fair, we only really started discussing pterosaur foraging traces in the early 2000s and prospects of terrestrial foraging were considered poor to non-existent by most workers at that time. Perhaps the concept of pterosaur foraging traces were just not on the radar for many researchers in 2003. In any case, these gauges do look similar to marks thought to record sweeping pterosaur beaks at other track sites (Lockley and Wright 2003), so the El Palillal gouges might indeed represent similar phenomena. If this is the case, this specimen might have some important implications for how we interpret azhdarchid pterosaur habits. Speaking of which...

What these fossils might indicate about azhdarchid pterosaur palaeobiology

What we have here is the start of a fossil dataset on azhdarchid palaeoecology, comprising several indications of which animals ate them, possible indications of animals using their remains for shelter, and possible trace evidence of a foraging azhdarchid. The data is currently of a small sample size - only five specimens in total - but already offers several points of interest those wanting to understand azhdarchid habits.

Firstly, these occurrences show azhdarchids interacting with animals known to have existed in inland habitats. Several of the involved animals are entirely terrestrial, and the most aquatically adapted species yet known to have accosted an azhdarchid bone is a crocodyliform. Given that the latter group is a primarily freshwater lineage (and, indeed, the crocodyliforms in question lived in a freshwater deposit), this data is not inconsistent with this model. As regular readers may appreciate, an 'inland' palaeoecological signature is consistent with the 'terrestrial stalker' mode of azhdarchid life proposed in by myself and Darren Naish in 2008 (see below, also Witton and Naish 2008, 2015), where we argued that azhdarchids were not aquatic or marine adapted species, but much more at home in woodlands and plains, picking up small game with their oversize beaks. I see these palaeoecologically-relevant fossils as a test of this idea, and am happy to see that - so far - they are consistent with the model we proposed.

Azhdarchid terrestrial stalking, the infographic. From Witton and Naish 2015.

Secondly, if we do indeed have an azhdarchid foraging trace, proposals that azhdarchids can reach the ground with their jaw tips to feed are vindicated. Darren and I initially encountered some resistance to our 2008 'terrestrial stalker' proposal because some peers thought the azhdarchid neck would not permit the jaws to reach the ground. We argued that the jaw is so long that only minimal neck motion, or even just a bit of forelimb flexion, would see the jaws reaching the ground without problem. Azhdarchid beak scrapes would indicate that this was indeed the case and put this minor debate to rest.

Thirdly, the terrestrial stalker model might predict the presence of beak scrapes alongside azhdarchid footprints. After all, azhdarchids looking for or striking at prey may sometimes have held their jaw tips close to the ground - modern animals track prey in this fashion, after all. The potential existence of beak marks with the El Pallilal track may be 'smoking gun' evidence of azhdarchids foraging on the ground in the manner proposed in 2008. I stress that the El Pallilal tracks need re-examination to confirm these ideas, but, if I'm correct, these traces augment the 'terrestrial stalker' argument considerably.

One question I expect will be asked about these fossils will be whether they tell us much about azhdarchid susceptibility to predation. This is something which has been discussed at a technical level (see this blog post for details), but I'm not sure these specimens help us understand it further. My issue is that, although various ideas have been published on the likelihood of certain azhdarchid fossils representing scavenging or predation (mainly involving relative masses of the animals involved), I'm not sure we can account for the many factors behind the circumstances recorded in these remains. Those azhdarchids outlined above were certainly dead, or near death, when being chewed on as their bones show no signs of healing from the inflicted damage, but a number of scenarios could account for this. We know that modern predatory events are influenced by the health of the animals involved, the skills and behaviour of different predatory species and individuals, and circumstances of time and place. These are difficult to account for with those bitten and chewed azhdarchid fossils we currently have, so I prefer to have no opinion on these matters and focus on the significance of things we can say more positively.

OK, that's all for now. In the next, and concluding part of this series we'll see what the fossil record tells us about the rhamphorhynchiest pterosaur of all, Rhamphorhynchus muensteri.

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Monday, 14 March 2016

The Carnian/Norian Swiss pterosaur Caviramus schesaplanensis, one of the earliest species to take pterosaur anatomy to strange new places. Anyone else want to make puns about 'Cave-iramus' with this picture? No? Anyone...?

What happens when you take the innate weirdness of the Triassic Period - the evolutionary equivalent of the late 1960s in terms of experimentation, weirdness and tragic ends to interesting lineages - and multiply it by a pterosaur? One answer is the marvellously strange Late Triassic flying reptile Caviramus schesaplanensis. This animal is a relative newcomer to the pterosaur roster, first being described in 2006 based on an incomplete lower jaw from Switzerland (Fröbisch and Fröbisch 2006), before better material in the form of an incomplete skeleton turned up a few years later* (Stecher 2008). Several cranial and dental features indicate Caviramus had kinship with other European pterosaurs such as Eudimorphodon and Campylognathoides, but that it was a rather distinctive animal compared to even close relatives. This 1.35 m wingspan animal had a chunky skull, large crests on both upper and lower jaws, densely packed and gnarly teeth, long, slender wings and a robust, lengthy set of hindlimbs (Stecher 2008). Several of these features are 'extreme' variants of pterosaur anatomy common to other Triassic animals, and others represent the most pronounced development of anatomical traits of any pterosaur. They mean that, even a decade after the first remains of this animal were published, Caviramus still gives us a lot to think about as goes its life appearance, lifestyle and functional anatomy.

*This skeleton was described as the holotype of a new genus and species, Raeticodactylus filisurensis, but a number of recent workers have shown it to be very likely congeneric, if not entirely synonymous, with C. schesaplanensis. I'm treating the two as the same taxon here.

Fossil material referred to Caviramus. Above, the holotype jaw of Caviramus schesaplanensis, below, the incomplete holotype skeleton of "Raeticodactylus filisurensis", a taxon now considered by most to be Caviramus and perhaps even Caviramus schesaplanensis itself. From Fröbisch and Fröbisch (2006) and Stecher (2008).

One atypical aspect of Caviramus is the cranial crest. Sure, pterosaur headcrests are really not uncommon (it's perhaps fair to say seems crestless species are more unusual than crested ones) but they remain fairly rare in the Triassic and, even compared to much later pterosaurs, Caviramus is pretty well endowed in the crest department. Rather than the low midline ridge typical of many pterosaur bony crests, this structure projects rudely from the front of the snout to reach well above the rest of the skull. We don't see anything like this again in the pterosaur record until the lower Cretaceous, when the famously elaborate tapejarids adopted a similar configuration. As with these pterosaurs, it's likely a soft-tissue component extended the Caviramus crest tissues in some way. The posterior crest border of the only known Caviramus skull is badly preserved, but the lateral crest surfaces have the same fibrous textures as pterosaurs known to have large soft-tissue crest components, such as Pterodactylus and Tupandactylus. The most parsimonious interpretation of this is that Caviramus had a big soft-tissue crest too, and - if the bony crest portion is indicative of the soft-tissue extent, as seems apparent from some pterosaur fossils - it might have been quite a spectacularly adorned animal. Caviramus seems to represent one of the first experiments with this sort of outlandish headgear, there being only one other Triassic species which could rival it for crest development (Austriadactylus cristatus - see Dalla Vecchia et al. 2002) .

Multiple aspects of the Caviramus jaw are of interest. It was probably a powerful biter, and perhaps regularly consumed relatively tough prey such as invertebrates with thick exoskeletons or fish with hard scales. Such a diet is indicated by its blunted and worn tooth tips, and the enamel of the anterior teeth being strongly rugose - some readers may recall from a recent article that these features also occur in other specialists of hard prey, such as the giant, turtle-eating Cretaceous crocodylian Deinosuchus. Caviramus dentition is morphologically complicated and, again, indicates some specialisation. As with many Triassic pterosaurs, the teeth are differentiated into large, curving anterior fangs at the jaw tips and complicated, multicusped teeth behind these. These posterior teeth are so numerous and tightly packed that they actually sit obliquely in the jaw, overlapping one another to form a continuous, 'megaserrated' cutting surface. The depression of the jaw joint (another atypical feature for a pterosaur, and one that won't reappear until later in pterosaur evolution) permitted these teeth to occlude simultaneously rather than gradually, as occurs in animals with jaw joints level with the toothrow. Areas of Caviramus jaw muscle attachment are large, including a broadly expanded posterior lower jaw. The mandible and skull are not, as with some pterosaurs, delicately built from slender struts but comprised of deep bars and robust bone junctions. Cross sections of the Caviramus holotype jaw indicate that some cavities were present in the cranial skeleton, but that bone volumes were superior in at least some places. This was clearly an skull capable of delivering and withstanding forceful bites, and its configuration recalls some dinosaur species which are sometimes considered to be omnivorous (e.g. many small ornithischians). Maybe it was equipped with powerful jaws so that it could tackle a wide range of tough foods, including nutritious plant matter.

This strong skull and dental apparatus seems odd compared to the Caviramus humerus. This bone is about as long as expected for a pterosaur of this kind, but is distinctive for being very, very slender. Usually, pterosaur humeri are the most robust elements in the limb skeleton, but that of Caviramus is no wider than the more distal wing elements. So proportionally different is this bone that the shoulder and upper arm were probably much more slender in life than those of other pterosaurs. Quite what this means for Caviramus locomotion has not been looked into yet, and any attempt to assess this will be frustrated by the only known Caviramus humerus being somewhat imperfectly preserved. Still, it's known in enough detail to at least permit some basic comments.

One obvious question concerns what this humerus means for quadrupedal launch potential in this animal. A core basis to this hypothesis is that pterosaur humeri are much stronger than their femora (Habib 2008) but - going on a basic assessment of bone shape here - this is not obviously the case for Caviramus. We should not automatically default to assuming Caviramus was a bipedal launcher however, as it is small enough to not need atypically strengthened limb elements for launch. The limb bones of volant animals are expected to start showing strong signals of a launch strategy once their body mass hits 2 kg (i.e. it's above this mass where the humerus or femur strength starts to become disproportionately strong compared to the other limb elements - see Habib 2008 for details) but, at only 1.35 metres across the wings, Caviramus probably only massed a little over one kilo. I'm sure Caviramus did have a preference for a particular launch strategy (I'm not aware of any animals which can readily flip between quadrupedal and bipedal launch, except under special circumstances), but its size means we might need dedicated investigation to know which was more likely. Given that all other pterosaurs seem to be quad-launchers, my suggestion is to assume this as the null hypothesis for now until we have reason to assume otherwise.

Caviramus schesaplanensis skeletal reconstruction, somewhat updated from the original version in Witton (2013). Unknown elements based on Campylognathoides liassicus (see Padian 2008).

The slenderness of the Caviramus humerus might have impacted flight once airborne, too. Caviramus joins pterosaurs like Eudimorphodon and Campylognathoides in having very long wings, but lacks the stocky, probably powerfully muscled humeri of these species. These might have enabled Eudimorphodon et al. to be forceful fliers capable of rapid flapping, elevated speeds and high agility. But the delicately constructed humerus supporting a long distal wing in Caviramus might have curbed any potential for being a powerful flapper or aerial acrobat - its humerus would have been far more vulnerable to bending than those of other pterosaurs. This is not to say that it was purely restricted to gliding, however. Studies of avian wing construction show that their humeri do not have to be enormously strong to permit flapping flight (indeed, their humeral strength seems to scale more or less isometrically with body size - Witton and Habib 2010) and the fact Caviramus has a large deltopectoral crest to anchor flight musculature is a good indication it was an active flier. We might conclude that its long, slender wing bones were suited to soaring flight with limited flapping - long winged seabirds like gulls, albatross and terns might be a good modern flight analogue.

Readers familiar with the Caviramus illustration in my 2013 book Pterosaurs: Natural History, Evolution, Anatomy might note that the 2016 Caviramus (above) is rather differently posed. There's good reason for this - read on...

It's not only flight that this unusual humerus might have impacted: it might have imposed some restrictions for life on the ground. I don't think we should assume it was so slender that it was incapable of supporting the animal - again, we're not dealing with an enormously heavy species here - but its slenderness might have impacted how the limb functioned when grounded. Specifically, the apparent absence of an expanded elbow region suggests it had sprawling forelimbs. As noted above, the Caviramus humerus is a little imperfectly preserved in places, the distal end being the poorest bit, but the proximal ulna confirms that the elbow joint was not broad. Slender elbows have been interpreted as a signature of sprawling forelimbs in some pterosaurs, for two reasons (Witton 2015, also see this blog post). Firstly, they suggest that musculature operating the wrist was fairly reduced (remember that wrist action is controlled by muscles anchored around the elbow - see Fujiwara and Hutchinson 2012). This reflects both the stresses encountered when standing in a sprawling pose and practicalities of terrestrial locomotion. Walking animals need to clear their feet or hands from the ground when moving, and animals with erect limbs have to do this by collapsing limb joints to reduce the effective length of the limb. Sprawling animals can use motion of the upper limb bone (humerus or femur) to elevate the entire limb, and can therefore take steps without needing to collapse the distal joints. Secondly, slender pterosaur elbows seem to correlate with shoulder joints that prevent depression of the humerus below the horizontal, a bony stop at the base of the shoulder precluding adoption of erect forelimb poses in these species. Given what we see in other pterosaurs, then, we might assume Caviramus elbow morphology indicates it had sprawling forelimbs, although we really need better fossil material to verify this. As in all other pterosaurs, details of the femoral morphology indicate that the hindlimbs were likely held erect. The legs are long enough that even with a highly crouched forelimb the animal still looks very 'leggy', and, in spite of its sprawled forelimbs, it might have been a fast, sprightly terrestrial animal. I imagine long-legged, skinny-limbed Caviramus scuttling about the place might give some people the creeps if it were alive today - if there was ever a pterosaur that might indirectly trigger arachnophobia, it's this one.

Collectively, these points suggest Caviramus represents one of the oldest deviations from what might be considered a 'standard' pterosaur bauplan and perhaps one of the first developments of anatomical 'extremes' in the group, at least as goes skull and wing anatomy. What makes this remarkable is that Caviramus lived so soon after the pterosaurs evolved in the first place - it seems to have wasted no time in pushing the pterosaur skeleton to weird new places. Unfortunately, it's currently difficult to say how successful these experiments were. The Triassic pterosaur record is extremely poor, particularly outside of Europe, and it is difficult to provide any meaningful evaluation of the abundance or longevity of lineages from this period. In a broad sense, however, it might be significant that we don't find Caviramus-like humeri or jaws in the better understood pterosaur faunas of the Jurassic or Cretaceous. Maybe Caviramus represents a configuration that was unsuited to life beyond conditions of the Triassic or, alternatively, perhaps the more 'typical' anatomies of other pterosaurs were just more adaptable in the long run. Whatever the reality here, Caviramus is a good example of how diverse and adaptable pterosaur anatomy can be and how much we have to learn about the early history of this group.

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Who is this 'Mark Witton' chap?

Dr Mark Witton is a palaeontologist and palaeoartist, affiliated with the University of Portsmouth, UK. My technical research is focused on pterosaurs - Mesozoic flying reptiles - but my artwork has introduced me to a wide array of different fossil animals that are just as interesting. I work as a freelance author, consultant and artist: check out my work at MarkWitton.com, follow me on Twitter @MarkWitton, and browse my books here. Contact me at wittonprints[at]gmail.com. Due to volume of email I can't always reply to messages, but I do my best.